A thresholding process well known in the art uses a dither matrix to convert an image expressed in multilevel values (256-level values, for example) into binary values. Since this thresholding process renders each multilevel tone as a binary value (i.e., specifying a dot or no dot), the percentage of dots rendered in regions of an image having low-level values decreases. Therefore, problems can occur when an image subjected to the thresholding process includes fine lines represented by low-level values, because these fine lines may disappear when converted to binary values in the thresholding process due to the positional relationship of the lines and the dither matrix.
Therefore, a dithering technique has been proposed for cases in which the image subjected to the thresholding process includes fine lines. This technique involves setting the screen angle different from the angle of the lines.